Determination of the Acoustic Wave Flux in the Lower Solar Chromosphere

High-cadence observations of the quiet Sun have been obtained with the Transition Region And Coronal Explorer (TRACE) satellite in two UV passbands around 1600 and 1700 Å. The observational program has been optimized for the detection of high-frequency acoustic waves: the cadence is strictly regular, there is no data compression, and the exposure time is optimized for quiet-Sun regions. Significant intensity variations are detected up to 40 mHz frequency. Non-LTE radiation hydrodynamics simulations are performed in order to calculate the acoustic energy flux that the observed intensity variations correspond to. The derived acoustic energy flux spectrum at the formation height of the UV continua (about 400 km) is decreasing with frequency. For frequencies above 40 mHz we derive an upper limit. The integrated acoustic energy flux is 255 W m^-2 in the frequency range 5-50 mHz. This is lower than what is needed to balance the radiative losses from the quiet-Sun chromosphere by at least a factor of 10. The major uncertainty in the analysis is the possibility of high-frequency power with spatial scales smaller than the resolution element of TRACE. We make estimates of this effect and find it unlikely that it is larger than a factor of 2. In the convection zone, where the waves are generated, the energy spectrum is rather flat and may have a peak at high frequencies. We also show that the sensitivity of the observations to high-frequency waves is directly given by the Fourier transform of the response function.